Plasma-enhanced chemical vapor deposition (PECVD or PCVD) is a process used to deposit thin films from a gaseous state (vapor) to a solid state. In the PCVD process, chemical reactions occur after creation of a plasma of the reacting gases.
Generally, in the field of optical fibers, multiple thin films of glass (e.g., glass layers) are deposited on the inside surface of a substrate tube. Glass-forming gases (e.g., doped or undoped reactive gases) are introduced into the interior of the substrate tube from one end (i.e., the supply side of the substrate tube). Doped or undoped glass layers are deposited onto the interior surface of the substrate tube. The gases are discharged or removed from the other end of the substrate tube (i.e., the discharge side of the substrate tube), optionally by the use of a vacuum pump. A vacuum pump has the effect of generating a reduced pressure within the interior of the substrate tube, such as a pressure of between 5 and 50 mbar.
Generally, microwaves from a microwave generator are directed toward an applicator via a waveguide. The applicator, which surrounds a glass substrate tube, couples the high-frequency energy into the plasma. In addition, the applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process. The applicator (and hence the plasma it forms) is moved reciprocally in the substrate tube's longitudinal direction. A thin glass layer is deposited onto the interior surface of the substrate tube with every stroke or pass of the applicator.
Thus, the applicator is moved in translation over the length of the substrate tube within the boundaries of a surrounding furnace. With this translational movement of the applicator, the plasma also moves in the same direction. As the applicator reaches the furnace's inner wall near one end of the substrate tube, the movement of the applicator is reversed so that it moves to the other end of the substrate tube toward the furnace's other inner wall. The applicator and thus the plasma travel in a back-and-forth movement along the length of the substrate tube. Each reciprocating movement is called a “pass” or a “stroke.” With each pass, a thin layer of glass is deposited on the substrate tube's inside surface.
Normally, a plasma is generated only in a part of the substrate tube (e.g., the part that is surrounded by the microwave applicator). Typically, the dimensions of the microwave applicator are smaller than the respective dimensions of the furnace and the substrate tube. The reactive gases are converted into solid glass and deposited on the substrate tube's inside surface only at the position of the plasma.
The passes increase the cumulative thickness of these thin films (i.e., the deposited material), which decreases the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate tube gets progressively smaller with each pass.
One way of manufacturing an optical preform via a PCVD process is disclosed in commonly assigned U.S. Pat. No. 4,314,833, which is hereby incorporated by reference in its entirety. According to this process, one or more doped or undoped glass layers are deposited onto the interior of a glass substrate tube using low-pressure plasma within the glass substrate tube. After the glass layers have been deposited onto the interior surface of the glass substrate tube, the glass substrate tube is subsequently contracted by heating into a solid rod (i.e., “collapsed”). In one embodiment, the solid rod may be externally provided with an additional amount of glass, such as via an external vapor deposition process or by using one or more preformed glass tubes, thereby obtaining a composite preform. One end of the resulting preform is heated, and optical fibers are obtained by drawing.
According to commonly assigned International Publication No. WO 99/35304 A1 and its counterpart U.S. Pat. Nos. 6,372,305 and 6,260,510, each of which is hereby incorporated by reference in its entirety, microwaves from a microwave generator are directed toward an applicator via a waveguide. The applicator, which surrounds a glass substrate tube, couples the high-frequency energy into the plasma.
Commonly assigned European Patent No. 1,550,640 and its counterpart U.S. Patent Publication No. 2005/0172902, each of which is hereby incorporated by reference in its entirety, disclose an apparatus for carrying out a PCVD process using choke having a specific length and width to minimize the losses of high-frequency energy during the entire deposition process, which reduces microwave leakage and leads to efficient energy consumption.
U.S. Pat. No. 4,741,747, which is hereby incorporated by reference in its entirety, relates to methods for reducing optical and geometrical end taper in the PCVD process. The regions of non-constant deposition geometry at the ends of the preform (taper) are reduced by moving the plasma in the area of at least one reversal point nonlinearly with time and/or by changing the longitudinal extent of the plasma as a function of time.
U.S. Pat. No. 4,857,091, which is hereby incorporated by reference in its entirety, relates to a PCVD method of making optical fibers whose refractive index profiles show specific peripheral and/or radial and/or axial optical modulation structures. Parameters are varied to influence (i) the uniformity of the material transport to the inner wall of the tube and/or the deposition yields of the glass over the tube circumference and/or (ii) the axial position of the local deposition zone with respect to the reactor producing the plasma.
United Kingdom Patent No. 2,068,359, which is hereby incorporated by reference in its entirety, relates to a PCVD process in which the plasma column is swept along the substrate tube by varying the power input to the device to effect direct formation of a glass layer along the swept and heated region of the tube.
Commonly assigned European Patent No. 2199263 and its counterpart U.S. Patent Publication No. 2010/0154479, each of which is hereby incorporated by reference in its entirety, disclose a PCVD process that can be used to minimize axial refractive index variations along a substrate tube by controlling the gas composition (primarily dopant composition) in the substrate tube as a function of the applicator (plasma zone) position.
U.S. Pat. No. 6,901,775, which is hereby incorporated by reference in its entirety, discloses an apparatus for internally coating a substrate tube via a PCVD process. The gas delivery unit includes an insert, which is inserted to prevent a disturbance in the gas flow that could induce a standing wave of a certain period and amplitude in the gas flow. According to this patent, the standing wave is, in turn, responsible for a non-uniform deposition within the interior of the substrate tube.
Commonly assigned European Patent No. 1,923,360 and its counterpart U.S. Pat. No. 7,981,485, each of which is hereby incorporated by reference in its entirety, disclose a PCVD process that provides uniform thickness and refractive index deposition in a substrate tube's axial direction. In this method, the furnace is moved reciprocally (e.g., 15, 30, or 60 millimeters) along a substrate tube's axial direction. The movement of the furnace is used to reduce the effect of what is believed to be non-uniform distribution of microwave power along the substrate tube's axial direction, which might be caused by microwave-applicator, position-dependent reflections of some of the microwave power (e.g., from the inner wall of the surrounding furnace). Such non-uniformity in axial microwave power can cause non-uniformity in axial deposition thickness and refractive index, which adversely affects optical-fiber quality parameters, such as attenuation, mode field width uniformity, and bandwidth uniformity.
Commonly assigned U.S. Patent Publication No. 2011/0247369, which is hereby incorporated by reference in its entirety, discloses a PCVD process in which the reaction zone moves back and forth in the longitudinal direction over a hollow glass substrate tube. An additional amount of a gas that includes a fluorine-containing compound is supplied to the interior of the hollow glass substrate tube when the reaction zone is located at or near the reversal point in order to reduce the incorporation of hydroxyl groups during the internal deposition process.
The foregoing notwithstanding, there is a need for an improved PCVD process that achieves more uniform glass deposition.